EP0843502B1 - Circuit de détection et prévention d'effet Larsen et un système de haut-parleur utilisant celui-ci - Google Patents

Circuit de détection et prévention d'effet Larsen et un système de haut-parleur utilisant celui-ci Download PDF

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Publication number
EP0843502B1
EP0843502B1 EP97119800A EP97119800A EP0843502B1 EP 0843502 B1 EP0843502 B1 EP 0843502B1 EP 97119800 A EP97119800 A EP 97119800A EP 97119800 A EP97119800 A EP 97119800A EP 0843502 B1 EP0843502 B1 EP 0843502B1
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EP
European Patent Office
Prior art keywords
howling
power
frequency band
identifying
band
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
Application number
EP97119800A
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German (de)
English (en)
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EP0843502A1 (fr
Inventor
Shigeo Ando
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Yamaha Corp
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Yamaha Corp
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Publication date
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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; ELECTRIC HEARING AIDS; PUBLIC ADDRESS SYSTEMS
    • H04R3/00—Circuits for transducers
    • H04R3/02—Circuits for transducers for preventing acoustic reaction, i.e. acoustic oscillatory feedback

Definitions

  • This invention relates to a howling detection and prevention circuit and a loudspeaker system employing this circuit.
  • a loud sound produced by the loudspeker is sometimes accompanied by howling.
  • a user manipulates a volume control while confirming presence or absence of howling to set the entire gain of radiation of acoustic power to a level at which howling is not produced or sets the entire gain after decreasing the gain of a particular frequency region by using a graphic equalizer or a notch filter. Adjustment of the gain is made relying upon the user's hearing.
  • the user usually hurries to the loudspeaker system and stops howling by lowering the volume level. In such a case, it takes time before howling stops and the user cannot avoid unpleasantness caused by howling during this time.
  • an object of the invention to provide a howling detection and prevention circuit capable of automatically detecting and preventing howling and a loudspeaker system employing the same circuit.
  • JP-A-60126998 discloses a control circuit which compares the level a signal of each band with the level of the signals of the bands adjacent to said band. When the difference between levels of both signals exceeds a certain level of threshold value, the generation of howling is decided. Then either one of attenuators corresponding to the band where the howling is generated is controlled to give a certain degree of attenuation. In case the howling is generated again at the same band, the howling is detected in the same way. Then a certain degree of attenuation is given to the attenuator corresponding to the relevant band. Thus the signals passing through said band are attenuated. The suppression of howling is continued until the total amount of attenuation reaches a certain level. Then the suppression of howling is finished when the number of bands where the degree of attenuation reaches a fixed value.
  • JP-A-60176313 in addition to band-pass filters for dividing into each frequency hand, a measuring circuit is provided, a magnitude of a signal is measured periodically extending over all frequencies, and a point in time when the magnitude of its signal is the smallest is detected.
  • a comparing circuit is operated, an output of measuring circuits for measuring a magnitude of a signal of each band is compared with a magnitude of a signal of both adjacent bands, and a band in which a difference of its magnitude has exceeded a set value is detected.
  • a controlling circuit gives a prescribed attenuation quantity of attenuators corresponding to the detected band.
  • a prescribed attenuation quantity is given to an attenuator for all frequency bands.
  • the signal supplied to an input terminal is supplied to plural band pass filters for frequency division, and the output is branched into two parts for each of the band pass filters.
  • One of these two branch signals is supplied to attenuators, and the other branch signal is supplied to average energy measuring circuits, respectively.
  • the average energy of each band signal obtained through said average energy measuring circuits is supplied to a control circuit.
  • the control circuit calculates the difference of the average energy levels before and after the corresponding band to the average energy of each band.
  • the outputs of these attenuators are added together and then delivered.
  • US-A-4,783,819 discloses an automatically controlled amplifier arrangement comprising a controlled amplifier unit (35), a detection unit (2) for detecting oscillations of the arrangement in the case of acoustic feedback during operation, and a control unit (3) which, in response to the detection unit (2), controls the gain of the amplifier unit (35).
  • the controlled amplifier unit comprises separate control means for separately controlling the gain for different frequency bands (1', 1", 1 "') below the howling limit. Prior to this a pre-adjustment is effected. Further, a circuit is proposed which increases the gain during preadjustment until oscillation occurs and which subsequently reduces the gain (if desired for each frequency band) until an adjustable margin below the howling limit is obtained (if desired for each frequency band).
  • an input signal is divided into frequency bands and thereafter power of each frequency band is computed.
  • the condition of howling is searched by sequentially changing the frequency band and, when there is a frequency band which satisfies the howling condition, this band is detected as a howling frequency band and prevention of howling is performed.
  • the howling detection and prevention circuit further comprises an entire gain adjusting section which adjusts entire gain of all of the frequency bands in accordance with the result of the identifying by the identifying section to prevent the howling.
  • said computing section computes power of each frequency band by computing moving averages with respect to each of the frequency bands which has been provided by frequency division on the basis of the predetermined sampling period.
  • said identifying section identifies presence or absence of howling on the basis difference between an absolute value of a power in the frequency band under identifying and a power of the frequency band in the vicinity thereof.
  • a loudspeaker system comprising a microphone, a howling detection and prevention circuit described above which receives a signal from the microphone as its input signal, an amplifying section which amplifies an output signal of the howling detection and prevention circuit, and a loudspeaker which is driven by an output of the amplifying section.
  • Fig. 1 shows an embodiment of a howling detection and prevention circuit according to the invention and a loudspeaker system incorporating this circuit.
  • a loudspeaker system 100 includes a microphone 101, a microphone amplifier 102 which amplifies an output signal of the microphone 101, an analog-to-digital converter 103 which converts the analog output signal of the microphone amplifier 102 to a digital signal, a howling detection and prevention circuit 1 which receives the digital output of the analog-to-digital converter 103 as an input signal, processes this input signal and supplies the result of processing to a digital-to-analog converter 104, a power amplifier 105 which amplifies the output signal of the digital-to-analog converter 104 in accordance with a gain which has been set at a desired value by an operator, and a loudspeaker 106 which is driven by the output signal of the power amplifier 105.
  • the howling detection and prevention circuit 1 includes various circuits such as a microcomputer, a signal processing chip, a memory and a timer. In Fig. 1, the internal structure of the circuit 1 is illustrated by blocks representing respective functions of the circuit 1.
  • a digital signal provided from the analog-to-digital converter 103 is applied to a band dividing filter section 11 of the howling detection and prevention circuit 1.
  • the band dividing filter section 11 consists of M (an integer including 2 and over) FIR (finite impulse response) bandpass filters or IIR (infinite impulse response) bandpass filters whose center frequencies are sequentially shifted.
  • the band dividing filter section 11 divides the input signal from the analog-to-digital converter 103 into signals of M frequency bands and, after imparting a predetermined gain to these signals, supplies these signals as signals F1, F2, Across FM to both an each band power computing section 12 and an adding section 13.
  • the each band power computing section 12 computes power values P1, P2, Across PM of the signals F1, F2, .
  • the adding section 13 adds the signals F1, F2, Across FM of the M bands together to obtain the signal of the entire bands and supplies the result of the addition to an entire gain control section 15.
  • the howling identifying section 14 identifies the state of occurrence of howling on the basis the power values P1, P2, Across PM of the signals in the respective bands and establishes, on the basis of the result of the identifying, a gain of each band which is used in the band dividing filter section 11 and also establishes a gain G for the signal of the entire bands which is used in the entire gain control section 15.
  • the entire gain control section 15 multiplies the sum signal of the entire bands with the gain G and supplies the result of the computation to the digital-to-analog converter 104.
  • the howling detection and prevention circuit 1 reduces, on the basis of the result of the identifying as to howling, a gain of a band in which howling has occurred or is likely to occur and thereby prevents occurrence of howling. In a case where howling remains unstopped despite the set gain for the band has been reduced, the howling can be stopped by reducing the entire gain G. By this arrangement, howling can be completely prevented even when an excessive gain has been set by the user.
  • the circuit block shown in Fig. 2 is a structure for computing a power of one frequency band in the each band power computing section 12 and the each band power computing section 12 has M blocks of the same construction.
  • the squarer 121a computes square value X0 2 of the input signal X0 and supplies the result X0 2 to the adder-subtractor 121b and to each input terminal of the N-tap shift memory 122.
  • the adder-subtractor 121b adds a computed value P obtained in the preceding sampling period k-1 and the square value X0 2 provided by the squarer 121a together and subtracts from the sum of this addition a value XN 2 (A) of the last stage of the N-tap shift memory 122 before shifting the memory (i.e., the output of the N-th tap) thereby to obtain a new computed value P.
  • the N-tap shift memory 122 sequentially shifts stored values of N memories and stores, as a value X1 2 (B), result of computation X0 2 of the squarer 121a in the current sampling period k.
  • Fig. 3 is a diagram showing a circuit block in the howling identifying section 14 for identifying whether howling has occurred or not or howling is likely to occur or not.
  • the howling identifying section 14 has, in addition to this circuit block, a circuit for selecting an input signal and a circuit for setting gains used in the band dividing filter section 11 and the entire gain control section 15 on the basis of the result of the identifying.
  • input signals Pm, Pm-1, Pm+1, Pm-2 and Pm+2 represent five signals consisting of a power signal Pm of a desired band in the power values P1, P2, Across PM of Fig.
  • comparison circuits 144, 145, 146, 147 and 148 perform comparison as to whether or not conditions Pm > TL1, Pm - Pm-1 >DIF1, Pm - Pm+1 > DIF1, Pm - Pm-2 > DIF2 and Pm - Pm+2 > DIF2 exist and output a result of comparison "0" (the condition is not satisfied) or "1" (the condition is satisfied).
  • TL1, DIF1 and DIF2 are reference values which are used for the comparison and set in conformity with actual conditions of use of the loudspeaker system.
  • An AND circuit 149 seeks a logical sum of results of comparison of all comparison circuits 144 to 148 and, when all of the conditions of comparison have been satisfied, outputs a signal "1" which represents the result of identifying that howling has occurred or is likely to occur.
  • Fig. 4 is a diagram which schematically shows an example of relation between the comparison reference values TL1, DIF1 and DIF2 and the input signals Pm, Pm-1, Pm-2, Pm+1 and Pm+2.
  • the example of Fig. 4 is illustrated on the assumption that the power signal Pm of the center frequency band has satisfied the above condition of identifying and therefore a howling states exists.
  • a signal of a frequency band in which howling has occurred or howling is likely to occur has a larger power than signals of frequency bands in the vicinity thereof and this relation is as illustrated in Fig. 4.
  • the signal which has satisfied the condition i.e., the power signal Pm of the center frequency band
  • the power signal Pm of the center frequency band has a peak with respect to signals of frequency bands in the vicinity thereof or not
  • the reference values DIF1 and DIF2 are normally set so that DIF1 becomes larger than DIF2.
  • DIF1 and DIF2 are normally set so that DIF1 becomes larger than DIF2.
  • This condition can be judged by comparing the power signal Pm with the reference value TL1, i.e., using not only identifying of difference value but also identifying of the absolute value.
  • both identifying of the absolute value i.e., as to whether the power signal Pm is larger than the reference value TL1 or not and identifying of the difference value, i.e., as to whether the power signal Pm is larger than the reference values DIF1 and DIF2 or not are made and, when both conditions are satisfied, it is judged that frequency band of the power signal Pm is in a howling state.
  • the howling identifying section 14 shifts the center frequency sequentially and performs judgment as to whether there is a frequency band which satisfies the howling condition and determines a band which satisfies the howling condition as the howling frequency band.
  • the howling identifying section 14 reduces the value of the gain used in the entire gain control section 15. By this operation, occurrence of howling which cannot be prevented by reducing the gain of the divided frequency band can be completely prevented.
  • identifying of howling can be made in real time and gain of each frequency band or entire gain can be adjusted automatically, so that prevention of howling which relied mainly upon the user's operation can be automatically performed.

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  • Health & Medical Sciences (AREA)
  • General Health & Medical Sciences (AREA)
  • Otolaryngology (AREA)
  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Acoustics & Sound (AREA)
  • Signal Processing (AREA)
  • Circuit For Audible Band Transducer (AREA)
  • Control Of Amplification And Gain Control (AREA)
  • Reverberation, Karaoke And Other Acoustics (AREA)

Claims (6)

  1. Circuit de détection et de prévention d'effet Larsen (1) qui reçoit la sortie d'un microphone (101) comme signal d'entrée et y détecte un effet Larsen, comprenant :
    des moyens de calcul de puissance (11, 12, 13) pour diviser la fréquence du signal d'entrée en une pluralité de bandes de fréquence et pour calculer par rapport à chacune des bandes de fréquence divisées la puissance de chacune des bandes de fréquence sur la base d'une durée d'échantillonnage prédéterminée;
    des moyens d'identification (14) pour identifier si de l'effet Larsen est présent ou non en utilisant la valeur de la puissance calculée de chaque bande de fréquence calculée par les moyens de calcul de puissance (11, 12, 13) en accord avec une différence (DIF1, DIF2) entre la valeur (TL1) de la puissance (Pm) dans la bande de fréquence par suite de la décision et de la valeur de puissance de la borne fréquence au voisinage de celle-ci, et avec une valeur absolue de la puissance de la bande de fréquence examinée ; et
    un moyen de réglage de gain (11, 14) pour, quand de l'effet Larsen a été détecté par suite de l'identification, régler le gain de la bande de fréquence dans laquelle l'effet Larsen a été détecté pour éviter l'effet Larsen.
  2. Circuit de détection et de prévention d'effet Larsen selon la revendication 1, comprenant en outre un moyen de réglage de gain d'ensemble (15) qui règle le gain d'ensemble de toutes les bandes de fréquence en accord avec le résultat de l'identification par le moyen d'identification (14) pour éviter l'effet Larsen.
  3. Circuit de détection et de prévention d'effet Larsen selon la revendication 1 ou 2, dans lequel les moyens de calcul (11, 12, 13) calculent la puissance de chaque bande de fréquence en calculant des moyennes mobiles par rapport à chacune des bandes de fréquence qui ont été fournies par division de fréquence sur la base de la période d'échantillonnage prédéterminée.
  4. Circuit de détection et de prévention d'effet Larsen selon l'une quelconque des revendications précédentes, dans lequel les moyens de calcul de puissance (11, 12, 13) comprennent, pour chaque bande de fréquence, des moyens de calcul de puissance de bande (12) pour calculer la puissance d'une bande de fréquence, chaque moyen de calcul de puissance de bande (12) étant de même structure et incluant un circuit d'actionnement (121) comprenant un moyen d'élévation au carré (121a), un additionneur-soustracteur (121b) et une mémoire à décalage (122) ayant une pluralité de prises (N).
  5. Circuit de détection et de prévention d'effet Larsen (1) selon l'une quelconque des revendications précédentes, dans lequel les moyens de calcul de puissance (11, 12, 13) calculent, sur la base d'une période d'échantillonnage prédéterminée, la valeur de la puissance correspondant aux moyennes mobiles d'un échantillon dans une période d'échantillonnage prédéterminée des bandes de fréquence divisées ; et
    dans lequel les moyens d'identification (14) identifient si un effet Larsen existe ou non en utilisant la valeur de la puissance calculée de chaque bande de fréquence calculée par les moyens de calcul de puissance (11, 12, 13).
  6. Système de haut-parleur (100) comprenant :
    un microphone (101) ;
    un circuit (1) de détection et de prévention d'effet Larsen selon l'une quelconque des revendications précédentes ;
    une partie d'amplification (105) qui amplifie le signal de sortie du circuit (1) de détection et de prévention d'effet Larsen (1) ; et
    un haut-parleur (106) qui est piloté par la sortie de la partie d'amplification (105).
EP97119800A 1996-11-13 1997-11-12 Circuit de détection et prévention d'effet Larsen et un système de haut-parleur utilisant celui-ci Expired - Lifetime EP0843502B1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
JP30219496A JP3152160B2 (ja) 1996-11-13 1996-11-13 ハウリング検出防止回路及びそれを用いた拡声装置
JP302194/96 1996-11-13

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EP0843502A1 EP0843502A1 (fr) 1998-05-20
EP0843502B1 true EP0843502B1 (fr) 2007-10-10

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US (1) US6252969B1 (fr)
EP (1) EP0843502B1 (fr)
JP (1) JP3152160B2 (fr)
DE (1) DE69738193T2 (fr)

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AU2004325701B2 (en) * 2004-12-16 2009-08-20 Widex A/S Hearing aid with feedback model gain estimation
US9749021B2 (en) 2012-12-18 2017-08-29 Motorola Solutions, Inc. Method and apparatus for mitigating feedback in a digital radio receiver

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Publication number Priority date Publication date Assignee Title
AU2004325701B2 (en) * 2004-12-16 2009-08-20 Widex A/S Hearing aid with feedback model gain estimation
US9749021B2 (en) 2012-12-18 2017-08-29 Motorola Solutions, Inc. Method and apparatus for mitigating feedback in a digital radio receiver

Also Published As

Publication number Publication date
DE69738193T2 (de) 2008-07-10
JPH10145888A (ja) 1998-05-29
EP0843502A1 (fr) 1998-05-20
JP3152160B2 (ja) 2001-04-03
DE69738193D1 (de) 2007-11-22
US6252969B1 (en) 2001-06-26

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